1. FIELD OF THE INVENTION:
This invention relates generally to electrical cables of the kind having a multiplicity of electrically conductive strands for conducting electrical energy through the cable. The invention relates more particularly to an improved multi-strand electrical cable which is uniquely constructed and arranged to suppress cable resonance and enhance the electrical power and signal transmission characteristics of the cable.
2. DISCUSSION OF THE PRIOR ART:
It has long been recognized that efficient electrical power and signal transmission applications requires the use of multi-conductor electrical cables; that is, electrical cables having a multiplicity of individual electrical conductors or wire strands. In some multi-strand cables, all of the strands are of the same cross-sectional size. Other multi-strand cables have strands of differing cross-sectional sizes. The cable strands are commonly twisted about the longitudinal centerline of the cable and encased within an insulating sheath which confines the body of strands radially of the cable. For reasons which are well understood in the electrical art and need not be explained in this disclosure, such multi-strand cables are characterized by enhanced electrical and mechanical properties relative to a single conductor cable. Among the foremost of these enhanced properties are improved fidelity and phase coherence in the case of audio and data signal transmission cables, and reduced electrical power losses and increased cable strength-to-weight ratio in the case of electrical power transmission cables or lines.
Further improvement in electrical power transmission lines was achieved a few years ago by replacing the copper conductors of multi-strand transmission cables with aluminum conductors. While aluminum has a lower electrical conductivity than copper, its density is sufficiently less than copper as to more than offset its lower conductivity and yield a conductivity-to-weight ratio greater than copper.
The present invention addresses one problem which is encountered to varying degrees in virtually all multi-strand electrical cables. This is the problem of reducing or eliminating resonance in the cables; that is, resonant vibration of the cable strands, which tends to occur in all multi-strand electrical cables, especially electrical power transmission lines. While this resonance problem is most pronounced in electrical power transmission lines, it may also occur, though to a much lesser extent, in audio and data signal transmission cables. In all cables in which it does occur, resonance has certain undesirable consequences which are discussed later. For this reason reduction or elimination of such cable resonance is highly desirable.
The phenomenon of electrical resonance in a multi-strand electrical cable is well known and understood by those versed in the electrical cable art and thus need be explained in this disclosure only in sufficient detail to enable a full and complete understanding of the invention. Suffice it to say that electrical current flow through a multi-strand cable produces like charges in and corresponding repulsion forces between the individual cable strands at every position along the strands. The current flow through cable strands of differing cross-sections,i.e. cross-sectional areas, and hence the electrical charges produced in the strands by such current flow, are directly proportional to the cross-sections of the strands. The effective repulsion force acting between adjacent strands of differing cross-sections is equal to or proportional to the charge in the smaller strand. This effective repulsion force urges the strands laterally apart against the action of an essentially resilient resisting force created by the elastic properties of and tension in individual strands, the mutual support between the strands, and the radial constraint of the cable sheath.
Fluctuations in the current flow through the cable strands produces corresponding fluctuations in the strand charges and thereby in the repulsion forces between adjacent strands. These fluctuations in the repulsion forces interact, in effect, with the resilient resisting forces on the strands in a manner which tends to cause relative back and forth lateral motion, that is, vibration, of the strands. The resulting relative vibratory movements or displacements of the cable strands vary their reactances and thereby introduce additional fluctuations and frequency components into the current flow through the cable.
In the case of multi-strand cables for A.C. electrical power transmission and signal transmission, current flow through the cables inherently fluctuates and thus tends to cause at least some degree of vibration of the cable strands in the manner discussed above. Cable strand vibration often occurs in multi-strand D.C. electrical power transmission cables also, however. This is due to the fact that any momentary spike or other momentary fluctuation in the D.C. current flow through the cable tends to cause relative displacement of the cable strands and produce a resultant change in the reactance of the strands. This change in reactance tends to counteract the original current fluctuation and thereby restore the cable strands to their original relation or positions. During this return of the strands to their original positions, the cable strands again undergo relative displacement which changes their reactance and introduces further fluctuations into the current flow through the strands. The end result of this action is vibration of the cable strands in much the same way as in an A.C. power cable.
If a strong frequency component or frequency components of the current flow through a multi-strand electrical cable approximate the natural frequency or frequencies of some or all of the cable strands, these strands tend to commence resonant vibration. This condition of resonant vibration of the cable strands is referred to in the art and in this disclosure as cable resonance. In an electrical power transmission line, this cable resonance produces an audible "hum" and has several undesirable effects. Foremost among these are the following. Resonance causes rubbing of the cable strands against one another which produces frictional heating and consumes electrical energy, thereby increasing the overall transmission line losses. Resonant vibration of the cable strands produces frictional wear and fatigue stress in the cable strands which weakens the strands and thereby the entire cable.
At the present time, such resonance in electrical power transmission cables is reduced somewhat by resonance dampers placed on the lines at intervals of about every two miles or so. While these dampers are effective to some extent, their damping effect is most pronounced in the immediate vicinity of the dampers and diminishes greatly or completely disappears in the regions between the dampers. Moreover, the dampers are relatively costly to procure, install, and maintain.
Although cable resonance is most pronounced and produces the most destructive effects in electrical power transmission lines, such resonance may also occur with undesirable consequences in electrical signal transmission cables, such as audio and data signal transmission cables. In these latter cable applications, while resonance may not produce physical destruction of the cable strands, such resonance can seriously degrade the transmitted signals by creating noise, distortions, and other aberrations in the signals.
A number of multi-strand signal transmission cable designs with various conductive strand arrangements have been devised for enhancing certain cable characteristics. Among the prior patents in this area, for example, are Brisson #4,538,023 and Cardas #4,628,151. Brisson discloses a multi-strand audio signal transmission cable of the kind commonly referred to as a Litz cable wherein relatively smaller diameter strands are disposed radially outwardly of relatively larger diameter strands, and are arranged to enhance the signal carrying capability of the cable. Cardas discloses a multi-strand cable wherein the cable strands have differing sizes and are relatively sized in accordance with the so-called "golden ratio progression" to further enhance the signal and power carrying capability of the cable. According to this golden ratio progression, the ratio of each strand cross-section to the cross-section of the next larger strand equals the ratio of the larger strand cross-section to the sum of the two strand cross-sections. Another patent of some interest in connection with multi-strand cables is Lejeune #3,413,799, disclosing a multi-strand reinforcing cable for automobile tires having strands of differing diameters.
None of the above-mentioned prior art patents or any other patents of which I am aware addresses the problem of reducing or eliminating resonance in a multi-strand electrical cable. While transmission line dampers have been devised to alleviate transmission line resonance, they are not totally satisfactory for the reasons stated earlier. Accordingly, there is a definite need for an improved multi-strand electrical cable which alleviates or eliminates such resonance. This invention provides such an improved multi-strand electrical cable.